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Gravitational Waves: Feeling Ripples in Spacetime

An artist’s impression of gravitational waves. Two heavy objects generate ripples in space as they orbit each other. The waves travel outwards and get weaker as they get farther from the source. This image by Penn State is licensed under CC BY-NC-ND 2.0.

Before the dawn of civilization, two giants spun. Floating out in the void of space, two black holes circled each other. With every rotation, they came ever so slightly closer. And finally, in an instant, they collided. In an event of unimaginable power, two black holes became one with a universe-shaking blast. And over a billion years later, humanity heard the boom.

A Wrinkle in Spacetime

Einstein’s general theory of relativity is one of the most successful scientific theories in history. It is the theory that describes gravity–why things fall down. Ever since Einstein published his theory in 1915, scientists have used it to make many fantastical predictions, including the existence of black holes. But one of the most astonishing predictions came from Einstein himself; in 1916, Einstein predicted the existence of something he called a gravitational wave.[1]

To understand gravitational waves, imagine a stadium crowd doing “the wave.” What exactly is the wave? No one person is the wave by himself; rather, the wave is a disturbance that moves through the crowd. A physicist would say that the crowd is the medium of the wave. Every wave has its medium: sea waves move through water, sound waves move through air, and seismic waves (earthquakes) move through the ground.

Gravitational waves visualized as ripples in a rubber sheet. As the waves go by, the sheet stretches and expands.“Wavy” by Frank Glowna is licensed under CC BY-SA 3.0

Einstein imagined a wave whose medium was space itself. Imagine the universe as a sheet of rubber; as a gravitational wave moves, it stretches and compresses the rubber. If we were to draw lines on the sheet, they would get longer and shorter as the wave went by. Einstein predicted that when very big, very heavy things moved very fast, they would create these waves and send them out in all directions.[2] One example of an object that could make gravitational waves is a black hole. Astronomers had predicted that pairs of black holes could form and rotate around each other at incredible speeds. Physicists calculated that when the black holes came close enough, they would merge into one, in an event of unimaginable magnitude. For a fraction of a second, two merging black holes release more energy than the rest of the universe combined – fifty times over.[3] That kind of energy is exactly what is needed to make gravitational waves strong enough to detect.

Scientists have wanted to measure gravitational waves ever since Einstein’s prediction. In 1974, astronomers gave us the first indirect evidence for gravitational waves by carefully looking at the orbits of extremely heavy stars.[1, 2] But scientists still wanted to directly measure the gravitational waves themselves. LIGO, or the Laser Interferometer Gravitational-Wave Observatory, was formed as a coalition of scientists working towards this goal.

How can a device measure the stretching of space itself? The idea behind LIGO is simple–it’s just a really high-tech ruler. Gravitational waves stretch things out in the direction they travel, just like the wave stretching out lines on the rubber sheet. LIGO is essentially a huge L-shaped machine made to measure two perpendicular lines very precisely. The two legs of the “L” represent the two lines. When a gravitational wave passes by, it stretches the two lines differently depending on its magnitude and direction.[2, 4, 5]

Imagine you wanted to measure how long a road was. You don’t have a long enough ruler, but you have a friend with a stopwatch. If your friend times how long it takes you to drive to the end of the road and back, while going at some constant speed, you can calculate the length of the road. LIGO uses this idea to measure the length of each leg of the observatory. LIGO shines a laser down the leg, reflects it off a mirror at the end, and times how long it takes the light to make the round trip. Since the speed of light is constant, the longer the leg, the more time the laser takes.[4, 5]

In practice, the difference in length between the legs is so tiny that no stopwatch in the world is accurate enough. So LIGO uses a trick called interference. Light is a wave with peaks and troughs. When two light waves are exactly lined up peak to peak and trough to trough, they add up to make the light brighter. Physicists say that the waves are in phase, interfering with each other to make a bigger wave. However, if the waves don’t quite line up, the troughs of one partially cancel out the peaks of the other and the light gets dimmer. These waves are out of phase, interfering with each other to make a smaller wave.

LIGO uses interference to compare the length of the legs. After the two laser beams travel along the legs, they are combined into one and shine onto a light detector. If the lengths are exactly the same, the beams are in phase, and make a bright dot. But if one leg is longer than the others, the waves are out of phase, and the dot gets a little darker. A device that lets lasers interfere with each other is called a laser interferometer, and this is where LIGO gets its name.

Two waves (blue and green) interfere to create a third wave (red). When the two waves’ peaks and troughs line up (the waves are in phase), the red wave gets bigger. On the other hand, when one wave’s peaks line up with the other’s troughs (the waves are out of phase), the waves cancel each other out. LIGO uses this phenomenon to measure length differences. “Wave Interference” by Natural Philo is licensed under CC BY-SA 4.0.

The World’s Most Sensitive Ruler

Building a precise enough ruler to measure gravitational waves was a gargantuan challenge. Detectable amounts of gravitational waves are only produced by gigantic cosmic events, and might happen billions of lightyears away. By the time it gets to us, a particularly strong gravitational wave would be so spread out that it might stretch LIGO’s legs by 0.00000000000000004 meters – an unimaginably tiny change.[6, 7] As a result, LIGO is the single most sensitive device in the world.[7] The name of the game is precision, and scientists used every trick in the book to increase it.

A technician installs a device inside one of LIGO’s legs. When LIGO is running, the entire leg is emptied of air and becomes a vacuum. Courtesy Caltech/MIT/LIGO Laboratory.

For starters, the longer the legs, the more their length changes when they’re stretched, so the legs are each 2.5 miles long.[7] However, even this isn’t nearly enough, so LIGO reflects the laser back and forth along the legs hundreds of times before measuring it. The light ultimately travels 700 miles before reaching the detector – longer than the distance from Atlanta to Philadelphia.[7] Anything LIGO’s laser touches could interfere with the measurement, so absolutely nothing touches the laser. LIGO’s entire 5-mile length is an ultrahigh vacuum, the second largest vacuum in the world (after the Large Hadron Collider).[8] The mirrors reflecting the laser have to be perfect to avoid scattering the light, so LIGO uses the smoothest mirrors ever created.[3] And this is just scratching the surface; every piece of LIGO was painstakingly designed and tested by teams of the best scientists in the world. And to be absolutely sure any detection was due to gravitational waves and not a local fluke, two LIGOs were built almost two thousand miles apart, one in Louisiana and another in Washington. Only a real gravitational wave would register the same signal on both.

Hearing Past the Noise

As a microphone gets more sensitive, it picks up the subtleties of a singer’s voice, but also the traffic outside. The same is true for any detector; more sensitivity means more noise, or random fluctuations unrelated to what is being measured. With a device like LIGO, noise is everything. The effect LIGO measures is so tiny that any bit of noise can drown it out completely – the device shaking, the wind swaying the building, traffic outside, even small earthquakes on the other side of the earth. So the noise must be quieted down in every way possible. “You play every trick you can to get the performance that you need,” said Brian Lantz. Brian is a Senior Research Scientist with the Stanford LIGO Group, part of the collaboration of scientists that made LIGO possible. “We’re always watching the baseline and we’re looking for changes in the baseline.” Scientists like Brian call the readings from the detector when there are no gravitational waves going by the baseline, which is just made up of noise. “The quieter we can make the baseline the easier it is to see disturbances,” he explained. “As you push down all the other noises more and more gravitational waves become apparent in your signals.”

Some of the equipment required to create LIGO’s laser. Courtesy Caltech/MIT/LIGO Laboratory.

The Stanford LIGO Group has been working for years to reduce LIGO’s noise. “We’ve been at it for a long, long time,” Brian remarked. One of the biggest contributions the Group has made is the creation of the laser used in previous iterations of LIGO. The laser is one of the most important sources of noise in LIGO. “If the laser frequency is changing the light coming back from the two arms will be a little bit different,” said Brian, “which is exactly what a gravitational wave looks like.” To solve these problems, the Group created a new type of laser. Describing the laser, Brian said, “The frequency coming out of the laser is much more stable, the lifetime of the laser is much longer – it’s pretty much better in every way.”

Turning Down the Shaking

Nowadays, the Group mostly focuses on isolating LIGO from external noise – shaking caused by seismic activity. “The mirrors for LIGO need to be about 1,000,000,000 quieter than the ground,” which is, as Brian explained, “a lot of isolation.” To accomplish this, Brian uses every stabilization technique he can. “There’s 7 layers of isolation between the ground and the mirror, and I work on the first 3,” Brian articulated. “First we do what’s called passive isolation – you take the table that you’re working on and you hang it from springs, and then if the ground shakes, then the table can’t really keep up and the table moves less than the ground does.” This technique is called passive because the table stabilizes itself, with no intervention required. “We also put computer control on the tables,” Brian continued, “and if we sense that the table starts moving one direction, we actively push back in the other.” This technique is called active because it requires outside intervention from the table’s computer to work. Another active technique deals with seismic activity directly. “We monitor the ground motion,” Brian described, “and if we notice that the ground is moving, say, to the right, we can tell the tables, ‘OK, move an equal and opposite amount to the left.’” All of these techniques work together to create extremely stable mirrors.

One layer of LIGO’s 7-layer isolation system. This is a passive layer, that stabilizes the mirrors using a mass hanging from a string. Courtesy Caltech/MIT/LIGO Laboratory.

Shi Tuck, an Undergraduate Researcher in the Group, is tackling a different part of the shaking problem. “The building gets a lot of wind, enough that it physically shakes the building, which is a bad sign when you have the world’s most sensitive detector housed inside,” explained Shi. The 50 mph winds sometimes force them to shut off the detector. According to Brian, that’s not good, since “the gravitational waves could come at any time, so you want to always be ready.”

“My job is seeing how can we mitigate the effects of these winds,” explained Shi. “Can we put up a barrier?” Shi has been modelling different wind barriers for the building to reduce shaking and tilt. Although simulations say her designs work, she is building them in wind tunnels to “see how the results translate to real life.” Shi is at the forefront of one of the most exciting science projects of our time, a remarkable achievement for being in just her second year at Stanford. To her, it’s about more than algorithms and designs. “I’m working on things that haven’t been done before and that’s really exciting,” she said. “I’m trying to understand the unknowns of the universe.”

A Primordial Boom

Scientists like Brian and Shi have been working on making LIGO a reality for decades. What started as just an idea in the 1960s became a mini-scale prototype in 1980 and led to the construction of LIGO facilities in 1997, which were recently upgraded to Advanced LIGO in 2015.[9] Researchers have spent countless hours calculating, designing, and dreaming of the day when humanity could finally hear the boom of gravitational wave.

Finally, in September 2015, that boom came. Both LIGO facilities detected a gravitational wave consistent with the merger of two huge black holes over a billion light-years away.[6] For this incredible discovery, leading members of the LIGO team including Rainer Weiss, Barry Barish, and Kip Thorne received the 2017 Nobel Prize in Physics.[2] This single event confirmed not only the existence of gravitational waves, but also the existence of black-hole pairs and the ability of black holes to merge, all of which had only been hypothesized before.[5] It was a momentous occasion in the history of science, and a triumph for anyone who’s ever looked up in awe at the stars.

The 2017 Nobel Prize in Physics was awarded to Rainer Weiss, Barry Barish, andKip Thorne “for decisive contributions to the LIGO detector and the observation of gravitational waves.”This image by Adam Baker is licensed by Flickr

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